Means are currently available for interrupting the circuit to an electric motor when the motor voltage is less than operational in order to prevent damage to the motor. U.S. Pat. No. 4,097,831 to C. L. Jencks et al. assigned to the common assignee of the instant invention, describes an undervoltage accessory for use with insulated case circuit breakers. The undervoltage facility is provided by a solenoid containing an armature which is spring-loaded for engaging the breaker tripping mechanism. The presence of a voltage of a predetermined value on the solenoid winding produces sufficient magnetic flux to hold the armature against the spring force. In the event that the voltage decreases in value, or becomes interrupted, the flux provided by the solenoid winding is insufficient to overcome the spring force causing the armature to extend and engage the breaker tripping mechanism.
It is disadvantageous for the undervoltage release trip mechanism to become activated upon a momentary undervoltage condition. This is especially true when the motor is involved in a complex manufacturing operation and time is taken to investigate the cause of the tripping occurrence. In some instances, the circuit breaker must be manually reset.
U.S. Pat. No. 4,011,484 assigned to the United States Government, describes an undervoltage release having an electrical reset for the circuit breaker. The automatic reset function is designed to start the motor when the voltage attains the correct operating value. An automatic reset function for electric motors however is not always desirable. In some instances, it could restart the motor at an inopportune time in a manufacturing process resulting in injury to personnel as well as to product. It would be more advantageous to delay tripping the motor supply upon the occurrence of temporary undervoltage conditions of short duration.
The purpose of this invention is to provide a magnetically bistable tripping mechanism in combination with a control circuit to delay the tripping operation during spurious undervoltage conditions and to trip the supply voltage circuit breaker when the undervoltage condition persists for a predetermined period of time.